The present invention relates to a demagnetizer for a magnetic head of a magnetic recording-reproducing device. It has particular application to tape recorders, magnetic card recorders, and magnetic disk recording-reproducing apparatus, especially cassette tape recorders.
During recording and reproducing, a magnetic head of a magnetic recording-reproducing device is gradually magnetized, and this magnetized head exerts a harmful influence upon the recording-reproducing characteristics. Accordingly, it is necessary to demagnetize the head periodically. A demagnetizer which produces a strong and definite alternating magnetic field is known, but it is necessary to move the demagnetizer manually so as to gradually decrease the magnetic field applied to the magnetic head. Such operation requires a great deal of skill.
A cleaning cassette having a permanent magnet in its case to demagnetize the head is disclosed in U.S. Pat. No. 3,647,990 issued Mar. 7, 1972, but it is difficult to demagnetize the head completely with such a device and it takes too much time to complete the demagnetizing operation. In any event, it is necessary to move the demagnetizer against the head in such prior devices.
It is an object of the present invention to provide a demagnetizer which can easily and completely demagnetize a magnetic head. Another object of the invention is to provide a demagnetizer which can demagnetize a magnetic head in a short time by use of a high frequency alternating magnetic field. Another object of the invention is to provide a compact demagnetizer which may be included in a tape cassette case and which does not require movement of the demagnetizer within the case away from the magnetic head to decrease the demagnetizing field.
These and other objects are attained in the present invention by a demagnetizer for a magnetic head comprising an electromagnet which is energized by a driving circuit so as to produce an alternating magnetic field which gradually decreases in intensity. A demagnetizer incorporated in a tape cassette case is preferably utilized for demagnetizing the magnetic head of a cassette tape recorder. The electromagnet is preferably energized by a driving circuit which includes an oscillator, the output of which is gradually decreased to produce the gradually decreasing alternating magnetic field. The oscillator may be controlled by a capacitor, the charge of which determines the output of the oscillator. When incorporated in a cassette case, the electromagnet is positioned adjacent to the opening therein for the magnetic head to be demagnetized. Th electromagnet is preferably resiliently held in place within the cassette case, and is coupled to a switch for controlling the energization of the electromagnet drive circuit.
The invention will be more completely understood by reference to the following detailed description.